Last night the BBC Persian service broadcast for the first time a very disturbing video of the attack by the Basij militia and riot police on Tehran University's campus just two days after the stolen election last June. The attack was one of the seminal events of Iran's post-election unrest in which the police broke locks and then bones as they rampaged through the dormitories, carted off more than 100 students and killed five.

The following day I went to Tehran University to report on the student protest. Students were demonstrating inside the university and behind the iron bars that separated them from outside. They carried a banner with the names of those who were arrested the night before. I talked to some students and took a picture of their banner. They also gave me the names of those five classmates who were killed in the campus attack.

A month later, I wrote a detailed story for the Guardian of what happened during the attack on the university campus, pieced together from interviews with students. But the new video shown on the BBC reveals more details from that shocking night.

For months, the Iranian authorities denied any incursion on to the campus took place but later supreme leader Seyed Ali Khamenei claimed that the unknown people – not the Basij or riot police – had attacked the students on the campus.

Surprisingly, the video shown on the BBC is not amateur footage but leaked from the police archive (how it came into the BBC's possession is unknown but may make an interesting story in itself). It begins moments before the attack when plainclothes officers used teargas and firearms against students. They later ransacked students' bedrooms and set beds on fire.

Azizollah Rajabzadeh, the former commander of Tehran police, is named in the video (by the cameraman) as the person behind the campus attack. Many people suspected Rajabzadeh but until this video appeared there was no confirmation. He recently left the Tehran police department for a different position in Tehran municipal office.

Under Iranian law, police, revolutionary guards and other militia are not allowed to enter universities – a legacy of the 1999 student riots. This is why many believe that even though Rajabzadeh ordered the attack he must have had prior approval from Khamenei and top commanders of the revolutionary corps.

The riot police behaved so brutally that even some of the Basij tried to stop them, as the video shows. Students were harassed verbally and physically when forced to lie on the ground. Some were bleeding profusely but the police continued to attack them. This was the point when five students were killed by being beaten with electric batons on their heads.

Later that night, students were made to stand in front of a dormitory block with plastic bags over their heads. The video their hands bound with plastic ties – known in Iran as "Israeli handcuffs" – and 46 of them were taken to the basement of the interior ministry on nearby Fatemi Street. It was there, on the building's upper floors, that the vote-rigging was going on, according to opposition supporters.

Another 87 were taken to a security police building on Hafez Street where they were tortured and mistreated. Their ordeal lasted 24 hours and before being released the students were ordered to put on fresh clothes (without bloodstains) supplied by the police.

Although news of the university campus attack circulated over the internet, many Iranians had not heard about until this video emerged and provided indisputable evidence. It also shows why, with so much brutality to hide, the regime is afraid of letting journalists report from Iran.

• The seven-minute clip shown on the BBC is an edited extract. A longer version has been posted on YouTube.